Determination of the effective water conductivity of sugar maple sapwood and white spruce heartwood under vacuum

摘要

A macroscopic approach based on the water potential concept is proposed to represent the movement of water in wood during continuous vacuum drying. In order to solve the flow equation, the effective water conductivity under vacuum must be known. A new apparatus is proposed to determine this moisture transport coefficient based on the instantaneous profile method, where moisture content profiles are established at different drying times, making possible the measurement of the moisture flux and driving force at a given position. One-dimensional moisture flow measurements were conducted through two sides of a cubic wood specimen under constant temperature and absolute pressure. The effective water conductivity function was established from green to dry conditions at 60deg C in the radial and tangential directions: at 8, 13, and 18 kPa for sugar maple (Acer saccharum) sapwood; and at 8 kPa for white spruce (Picea glauca) heartwood. The results show that the effective water conductivity decreases exponentially from green conditions to about 35% moisture content for sugar maple. Beyond this point, the effective water conductivity decreases more gradually with a decrease in moisture content for both sugar maple and white spruce. The effective water conductivity is generally higher in radial than in tangential direction for both species. The results obtained for sugar maple show that the effective water conductivity increases significantly as the pressure decreases. The effect of pressure can be explainedby the contribution of the apparent gas permeability. The contribution of the pressure potential to the total water potential can be neglected below fibre saturation point. The flux-gradient relationships obtained at given moisture contents are linear,confirming the validity of the flow equation based on water potential used in the present work.